This section provides background information related to the present disclosure, which is not necessarily prior art.
Acoustic insulator mats of the type described herein are commonly used in automobiles and other vehicles to reduce the amount of noise experienced by occupants. Such acoustic insulator mats, also referred to as noise attenuation systems, are typically made of foam and/or “shoddy” material, built up for lightweight constructions. Shoddy is typically made of cotton or other non-woven fibrous materials that absorbs noise and a non-liquid, binder bi-component that holds the cotton or other non-woven fibrous materials together. Accordingly, acoustic insulator mats are typically molded sheets that can conform to substantially any vehicle component shape.
Vehicle manufacturers often install acoustic insulator mats in multiple locations inside and outside a passenger compartment of a vehicle. Because acoustic insulator mats are flexible, they are typically retained and supported on a wall of the vehicle at multiple locations. One such acoustic insulator mat is disclosed by Gahlau et al. in U.S. Pat. No. 4,655,496 entitled “Motor Vehicle Noise Insulation.” As shown in this reference, such acoustic insulator mats are often applied in a blanket form to cover the walls of a vehicle such as the engine firewall and transmission tunnel in order to attenuate engine noise and road noise in the passenger compartment of the vehicle. This reference also shows that typical acoustic insulator mats often require pass-through openings that accommodate various penetrating members, such as a steering column, brake and accelerator linkages, shift linkages, electrical wiring, and ducting.
The location of acoustic insulator mats within the vehicle and the material that is used for these acoustic insulator mats typically varies between two different schools of thought. Some vehicle manufacturers add acoustic insulator mats to vehicles to reduce the amount of sound that enters the passenger compartment. Such acoustic insulator mats typically include sound barrier films attached to shoddy layers. Such sound barrier films exhibit high transmission loss and reflect noise away from the passenger compartment (i.e. sound barrier films block a large amount of incoming noise and prevent this incoming noise from entering the passenger compartment). By way of example, sound barrier films may be made from sheets of polypropylene, polyvinylchloride, polyethylene, ethylene vinyl acetate, or other thermoplastic materials. One problem with this approach is that the entire passenger compartment cannot be sealed/shielded by acoustic insulator mats and therefore sound will inevitably enter the passenger compartment. For example, sound typically enters the passenger compartment through the vehicle's windows, regardless of whether the windows are open or closed. Because sound barrier films are designed to reflect sound, acoustic insulator mats with sound barrier films typically do not absorb sound well. Any noise that does make its way into the passenger compartment is typically reflected back and forth in the passenger compartment by the sound barrier film and is not attenuated or absorbed by the acoustic insulator mat.
Other vehicle manufacturers add acoustic insulator mats to vehicles to absorb noise within the passenger compartment. Such acoustic insulator mats are made entirely of highly sound absorbent materials such as shoddy. The ability of such acoustic insulator mats to absorb and dissipate noise contained within the passenger compartment typically increases with mat thickness. Accordingly, a disadvantage of this approach is that well performing acoustic insulator mats add significant weight to the vehicle, which negatively impacts the performance and fuel economy of the vehicle. Another disadvantage with this approach is that highly sound absorbent materials also typically have low transmission loss. Therefore, little sound is prevented from entering the passenger compartment by the acoustic insulator mat so there is more noise in the passenger compartment that requires attenuation. Therefore, a need exists for an acoustic insulator mat that blends these two approaches, which traditionally have been viewed as mutually exclusive due to material limitations.